1. Field of the Invention
The present invention relates to a production method for complex bearings which are equipped with a cylindrical sintered bearing member and a cylindrical outer member into which the sintered bearing member is integrally fitted. For example, the complex bearings are bearing complex gears (for example, planetary gears) and bearing units. Each bearing complex gear is equipped with a sintered bearing member and a gear member (outer member) having a center hole into which the sintered bearing member is fitted. Each bearing unit is equipped with a sintered bearing member and a housing (outer member) into which the sintered bearing member is fitted and fixes.
2. Description of the Related Art
A planetary gear moves around a sun gear while engaging with the sun gear and rotating on its rotational axis. Since the planetary gear engages with the sun gear and transmits a power thereto, it is required to have a corresponding strength. In addition, it is required to have a low friction to the rotational axis and have a good sliding property thereto. Thus, in order to improve a sliding property of a planetary gear to a rotational axis, for example, Japanese Examined Utility Application Publication No. S61-139349 has proposed a method in which a bearing member of oil-impregnated alloy member is fitted into a gear member which is used as a peripheral portion having teeth formed thereat. The oil-impregnated alloy member may be an oil-impregnated sintered alloy.
A sintered alloy made by powder metallurgy is easily formed into a near net shape at relatively low cost, thereby being used as a material of planetary gear. In the above combination of the gear member and the bearing member, sintered alloys are made by selecting raw materials, density ratios, sintered conditions, and the like, based on required properties of the respective members, and they are used such that a Fe-based sintered alloy superior in strength property is used for the gear member and an oil-impregnated sintered bearing is used for the bearing member. As a result, a planetary gear is produced. In the planetary gear, it is necessary to strongly integrate the gear member and the bearing member with each other. However, when an adhesive material is used for adhesion of the members, since the adhesive material is absorbed in pores of the members, the adhesion of the members cannot be strong. Thus, a method proposed by Japanese Unexamined Patent Application Publication No. H7-238880 has been conceived to be advantageous, in which a bearing member is press-fitted into a center hole of a gear member.
FIGS. 13A and 13B show one example of production method for obtaining a planetary gear by fitting a sintered member into a bearing member. In this method, first, as shown in FIG. 13A, a gear member 301 is mounted onto a die 402 into which a columnar core rod 401 is slidably inserted. An upper end portion of the core rod 401 penetrates a fitting hole 301a which is formed at a center of the gear member 301 and has a circular cross section. A cylindrical bearing member 302 is fitted into an upper end portion of the core rod 401 which projects from the gear member 301. In this case, the bearing member 302 has an outer diameter which is slightly larger than an inner diameter of the fitting hole 301a and thereby allows interference fitting of the bearing member 302 into the gear member 301. Next, as shown in FIG. 13B, the bearing member 302 is pressed into the fitting hole 301a of the gear member 301 by a cylindrical punch 403. The bearing member 302 is guided by the core rod 401, and is press-fitted into the fitting hole 301a of the bearing member 301, so that a peripheral surface of the bearing member 302 tightly contacts an inner peripheral surface of the fitting hole 301a. As a result, the gear member 301 and the bearing member 302 are integrated with each other. In addition, the inner peripheral surface of the bearing member 302 tightly contacts the core rod 401, so that an inner diameter of the bearing member 302 is adjusted.
In the combination method by the press-fitting as shown in FIGS. 13A and 13B, as shown in FIG. 14B, the peripheral surface of the bearing member 302, which is press-fitted into the gear member 301, is cut by an upper edge of the fitting hole of the gear member 301, so that burrs 302a are often generated. Due to this, it is necessary to remove the burrs 302a. In addition, since the peripheral surface of the bearing member 302 flows in the above manner, the contact area of the peripheral surface of the bearing member 302 with respect to the inner peripheral surface of the fitting hole of the gear member 301 decreases, and the degree of contact is deteriorated. Thus, the expected fixing strength cannot be obtained. Due to this insufficient fixing strength of the bearing member 302 with respect to the gear member 301, slip between the members 301 and 302 occurs in practical use as a gear, and a problem occurs in transmission of force.
An oil-impregnated sintered bearing, which is made from a porous sintered alloy and has lubricating oil impregnated thereinto, can be used without oil supplying for a long time period, it is superior in durability at high temperatures, and it generates low noise. Therefore, instead of a ball bearing, the oil-impregnated sintered bearing has been widely used as a bearing for a rotational shaft. For example, in spindle motors (which are used in information devices) and fan motors, the above oil-impregnated sintered bearing is fitted and fixes into a housing which is composed of a cut product of ingot material (for example, brass and stainless) or is composed of a die-cast product of Zn or Al. As a result, the oil-impregnated sintered bearing and the housing are assembled as a bearing unit. Next, lubricating oil is impregnated into pores formed in the bearing unit, and it is held therein. Thus, an oil film is formed between an inner peripheral surface of shaft hole and a rotational shaft which is slidably inserted into the shaft hole, and lubricating condition therebetween can be maintained for a long time period (see FIGS. 2 and 1 of Japanese Unexamined Patent Application Publication No. H9-96314, FIGS. 4, 5, 6 and 1 of Japanese Unexamined Patent Application Publication No. H7-332363, and FIG. 1 of Japanese Patent No. 3686665).
In the above bearing unit, when the degree of contact of the bearing member with respect to the housing is insufficient, the bearing member (oil-impregnated sintered bearing) may project from the housing, and it may fall therefrom. Due to this, the fitting hole of the housing, into which the bearing member is fitted, is machined to have a high size precision and a small surface roughness in order to improve close contact of the fitting hole of the housing and the peripheral surface of the bearing member. When an adhesive material is used as a method in which the bearing member fixes in the housing, the adhesive material is absorbed in pores of the bearing member, so that strong fixation cannot be obtained. Therefore, a method for press-fitting of the bearing member into the housing is conceived to be advantageous.
In order to press-fit a bearing member into a housing, for example, a die apparatus shown in FIGS. 15A and 15B is used. The die apparatus is equipped with a die 590, a core rod 591, upper and lower compression punches 592 and 593, and a supporting punch 594. The die 590 has a cylindrical die hole 590a into which a housing 580 is fitted. The core rod 591 is disposed at a center of the die hole 590a of the die 590. The upper and lower compression punches 592 and 593 are used for compressing a sintered bearing member 585 of a sintered compact which is inserted into the housing 580. The supporting punch 594 supports the housing 580. The housing 580 is cylindrical and has a fitting hole 580a which is formed at a center thereof and into which the sintered bearing 585 is fitted. The cylindrical bearing member 585 before press-fitting has an outer diameter which is slightly larger than an inner diameter of the housing 580 (that is, diameter of the fitting hole 580a) and thereby allows interference fitting of the bearing member 585 into the housing 580. A shaft hole 585a has an inner diameter allowing insertion of the core rod 591 into the shaft hole 585a by sliding.
As shown in FIG. 15A, the bearing member 585 is coaxially disposed above the core rod 591. As shown in FIG. 15B, while the bearing member 585 is guided by the core rod 591 which slides on the shaft hole 585a and is inserted thereinto, the bearing member 585 is pressed into the fitting hole 580a of the housing 580 by the upper punch 592. As a result, a bearing unit is obtained. The bearing member 585 is press-fitted into housing 580, so that a peripheral surface 580a of the bearing member 585 tightly contacts an inner peripheral surface 580a of the housing 580. As a result, the housing 580 and the bearing member 585 are integrated with each other. In addition, the inner peripheral surface of the bearing member tightly contacts the core rod, so that an inner diameter thereof is adjusted.
In the method as shown in FIGS. 15A and 15B, the degree of contact of the bearing member 585 with respect to the housing 580 is uneven, and the fixing strength may be low at a portion thereof. According to analysis of the reason by the inventors, as shown in FIGS. 16A and 16B, the peripheral surface of the bearing member 585 fitted into the housing 580 is cut by an upper edge of the fitting hole 580a of the housing 580, so that burrs 302a are generated. In addition, since the peripheral surface of the bearing member 585 is cut, the contact area of the peripheral surface of the bearing member 585 with respect to the inner peripheral surface of the fitting hole 580a decreases.
Japanese Patent No. 3686665 has proposed a method in which a sintered bearing member, which has an outer diameter slightly smaller than a diameter of fitting hole of housing, is fitted into the housing in a clearance fit condition, an adhesive material is filled between the sintered bearing member and the housing, and they fix to each other. In particular, this method is characterized in that impregnation of resin into pores of the sintered bearing member is performed, the pores are sealed, an inner portion unit is subjected to vacuum suction, and the adhesive material, which falls in drops at end portion of the clearance, reach the overall of the clearance. However, in this method, since the pores of the sintered bearing member are sealed by the resin impregnation, the fixing property is good, but oil film formation action, which caused by lubricating oil supplying from the pores of the oil-impregnated sintered bearing, is deteriorated. Due to this, it is difficult to use this method for typical bearing units.